The metals copper, silver, and gold, found in Group IB of the Periodic Table, often are employed as catalysts in the production of large quantities of valuable chemical compounds, especially aldehydes. Citral and lilestralis are two important examples of aldehydes produced using Group IB metal catalysts.
Lilestralis (p-tert-butyl-alpha-methyldihydrocinnamaldehyde, formula 1), an important fragrance ingredient, can be produced by liquid phase dehydrogenation of liliol in the presence of a group IB metal catalyst. ##STR1## Generally, oxygen is not used in the conversion of liliol to lilestralis because the lilestralis product is susceptible to further oxidation to p-tert-butylbenzyl methyl ketone, which cannot be conveniently removed from the reaction mixture.
Citral (3,7-dimethyl-2,6-octadienal, formula 2) is an important ingredient in flavoring and perfumery products, especially those involving citrus formulations, and is an intermediate in the manufacture of ionones, methyl ionones, and vitamin A. ##STR2## Citral can be produced by reacting unsaturated terpenic alcohols such as geraniol and/or nerol with an oxygen-containing gas at an elevated temperature in the presence of a copper catalyst. For example, British Patent Specification 1,381,587, which is incorporated herein by reference, discloses a method for preparing citral using as a catalyst either wire gauze formed from a group IB metal or the wire gauze plated with group IB metal. Alternatively, the metal can be carried on a non-acidic support.
High surface area copper catalysts such as Raney copper catalysts or oxidized and fresh ultrafine copper powders [Sakka, et al., "Sintering and gas desorption characteristics of copper ultrafine powders", Materials Transactions, vol. 31, no. 9, 802-809 (1990)]generally are believed to be unsuitable for vapor phase dehydrogenations, because their fine nature is not conducive to the free flow of gases present in such reactions. Raney catalysts can, however, be used in certain fixed bed operations. Tomsett, et al., Applied Catalysis, 1987, 35, 321, disclosed that a Raney catalyst having a high surface area of copper minimized problems associated with pore diffusion. These Raney catalysts are believed to be unsuitable in processes that cannot tolerate the impurities that are co-generated during production of the catalyst.
Although copper catalysts have been employed in preparing aldehydes from alcohols, the high temperatures and oxidizing conditions associated with these conversions often cause copper substrates to become brittle and degrade. For example, copper spheres can lose approximately 50% of their catalytic reactivity within one to two weeks under such conditions.
A number of catalysts based on group IB metals are known in the art. For example, U.S Pat. No. 4,480,136 discloses a process for preparing norbornyl oxyacetaldehyde using a silver or copper chromite catalyst.
U.S. Pat. No. 4,816,606 discloses a continuous process for preparing aldehydes and ketones wherein an alcohol is oxidized with oxygen in the presence of a supported catalyst composed of an inert carrier having a smooth surface and from 0.1 to 20% by weight, based on the weight of the carrier, of an active layer of copper, silver and/or gold. The active metal is applied to the inert material to provide a smooth, abrasion-resistant coating.
U.S. Pat. No. 4,503,261 discloses a process for preparing glyoxal wherein ethylene glycol, oxygen, and an inert gas are passed over a catalyst consisting of one or more layers of silver and copper crystals having a particle size of from 0.1 to 2.5 mm. The catalyst contains from 10 to 45% by weight of copper crystals and from 55 to 90% by weight of silver crystals based on the total weight of all the catalyst particles.
U.S. Pat. No. 4,560,759 discloses a process for preparing pyrroles comprising reacting ammonia or an amine with a diol in the presence of a supported catalyst containing copper, silver, zinc, palladium, nickel, cobalt and/or platinum and/or compounds of these metals. The supported catalyst comprises from 0.2 to 25% by weight active metal based on the weight of the support.
U.S. Pat. No. 4,851,584 discloses a process for preparing carbonyl compounds wherein alcohols are oxidized with an oxygen-containing gas in the presence of a catalyst containing copper and silver or alloys in which the mass fraction of copper or silver is more than 90%. The catalysts are prepared by coating an inert material with the metal by known methods such as flame coating, plasma coating or sputtering.
U.S. Pat. No. 4,814,513 discloses a process for preparing carbonyl compounds by oxidizing an alcohol with an oxygen-containing gas in the presence of a catalyst comprising copper or silver and a phosphorus compound that is volatile under the conditions of the reaction. Metallic copper or silver, copper or silver alloys, or compounds with metals or non-metals are said to be suitable as catalysts. The catalyst can be applied to an inert carrier and can be subjected to reducing treatment before use, if required.
Accordingly, a method for producing aldehydes is needed in which catalytic reactivity can be extended and the yield and purity of the final product can be improved. There is also a need for catalysts that are resistant to accelerated erosion under a variety of conditions.